Chromatic aberration is an important photographic phenomena to grasp. It might sound difficult, but it is easy enough. Imperfections in the lenses create the fringes, that are seen as magenta and blue-green fringes. It comes in two varieties: 1. The individual colors do not focus on the same sensor plane. 2. The individual colors produce images of different size. In the following article we will look in depth at the phenomena of chromatic aberration and how to avoid or solve it.
First we have to understand what refractive index is. Light changes its direction when it passes through a medium like the glass of the lenses. For example light may hit the lens at a 90 degree angle, but leave the lens at an 80 degree angle. Chromatic aberration arises because the different colors of light have different refractive indexes. For example blue might leave the lens at 79 degrees while red might leave at 81 degrees. This produces what is known as longitudinal chromatic aberration where you see thin magenta fringes. Since green is in-between red and blue it is used to focus the lens. Thus the red and blue are slightly out of focus which creates the magenta (red+blue) fringes.
Transverse chromatic aberration arises when light does not enter the lens at 90 degrees, but from a different angle. Here the individual colors focus on the same sensor plane, but not at the same spot, thus the red image will be larger than the green and blue image, and blue smaller than red and green.This also creates colored fringes, but now both a magenta and a blue-green one. It is in the interest of lens manufacturers to avoid chromatic aberration, but since it is in the nature of light, it is hard to eliminate.
Both types of chromatic aberration produce color fringes, but of a different sort. Longitudinal aberration produces magenta fringes around objects and is spread evenly throughout the image. Transverse aberration is absent at the center of the image, but grows in intensity towards the corners. Longitudinal chromatic aberration is most pronounced in wide aperture lenses. It can be reduced by using a small aperture. Transverse chromatic aberration is most pronounced in telephoto lenses. However, lenses can be designed in many ways. The so called achromatic lenses are by far the most popular with minimal chromatic aberration. More rare are the superachromatic and apochromatic lenses, that virtually eliminate chromatic aberration. Digital images tend to show more chromatic aberration than film for some reason. This may be because the sensors are more sensitive to ultraviolet and infrared light, which are at the outer edge of the spectrum where aberration is most pronounced.
Software can fix cromatic aberration. Longitudinal chromatic aberration is somewhat corrected by sharpening the red and blue channels; the green channel is used to focus the image and is sharp. Transverse chromatic aberration is satisfactorily corrected by radially enlarging the blue channel image and radially reducing the red channel image.
A different kind of chromatic error is the dreaded purple fringe. It appears along hard contrast edges when photographing something against a hard back light, or when photographing a light source against a dark background.The purple fringe invades the dark area. Purple fringes are sensor errors, whilst chromatic aberrations are lens errors. It is very difficult to correct purple fringes with software since it is really an overflow of light from one sensor to the surrounding ones, and is not a simple geometric error like transverse chromatic aberration. Also the original color is usually suppressed. Software can reduce the color of the purple fringe to a grayish tone. At best the local color is not completely eradicated by the purple fringe and can be reconstructed.
First we have to understand what refractive index is. Light changes its direction when it passes through a medium like the glass of the lenses. For example light may hit the lens at a 90 degree angle, but leave the lens at an 80 degree angle. Chromatic aberration arises because the different colors of light have different refractive indexes. For example blue might leave the lens at 79 degrees while red might leave at 81 degrees. This produces what is known as longitudinal chromatic aberration where you see thin magenta fringes. Since green is in-between red and blue it is used to focus the lens. Thus the red and blue are slightly out of focus which creates the magenta (red+blue) fringes.
Transverse chromatic aberration arises when light does not enter the lens at 90 degrees, but from a different angle. Here the individual colors focus on the same sensor plane, but not at the same spot, thus the red image will be larger than the green and blue image, and blue smaller than red and green.This also creates colored fringes, but now both a magenta and a blue-green one. It is in the interest of lens manufacturers to avoid chromatic aberration, but since it is in the nature of light, it is hard to eliminate.
Both types of chromatic aberration produce color fringes, but of a different sort. Longitudinal aberration produces magenta fringes around objects and is spread evenly throughout the image. Transverse aberration is absent at the center of the image, but grows in intensity towards the corners. Longitudinal chromatic aberration is most pronounced in wide aperture lenses. It can be reduced by using a small aperture. Transverse chromatic aberration is most pronounced in telephoto lenses. However, lenses can be designed in many ways. The so called achromatic lenses are by far the most popular with minimal chromatic aberration. More rare are the superachromatic and apochromatic lenses, that virtually eliminate chromatic aberration. Digital images tend to show more chromatic aberration than film for some reason. This may be because the sensors are more sensitive to ultraviolet and infrared light, which are at the outer edge of the spectrum where aberration is most pronounced.
Software can fix cromatic aberration. Longitudinal chromatic aberration is somewhat corrected by sharpening the red and blue channels; the green channel is used to focus the image and is sharp. Transverse chromatic aberration is satisfactorily corrected by radially enlarging the blue channel image and radially reducing the red channel image.
A different kind of chromatic error is the dreaded purple fringe. It appears along hard contrast edges when photographing something against a hard back light, or when photographing a light source against a dark background.The purple fringe invades the dark area. Purple fringes are sensor errors, whilst chromatic aberrations are lens errors. It is very difficult to correct purple fringes with software since it is really an overflow of light from one sensor to the surrounding ones, and is not a simple geometric error like transverse chromatic aberration. Also the original color is usually suppressed. Software can reduce the color of the purple fringe to a grayish tone. At best the local color is not completely eradicated by the purple fringe and can be reconstructed.
About the Author:
This article is based on this: chromatic aberration I and this: chromatic aberration II. Also published at What is Chromatic Aberration?.
Post a Comment